This paper analyzes yield trends in 30 long-term experiments (LTEs) conducted at 24 sites with intensive rice monoculture or rice-upland crop systems in tropical and subtropical regions of Asia. The objectives of the analysis were to (1) investigate how widespread yield declines are in experiments with constant levels of input and (2) study the cross-sectional relationship
between the magnitude of yield trends and the initial yield level in the experiment. In rice±rice (±rice) systems, yields in the most favorable climatic season (dry season) have remained stable in many LTEs conducted outside of IRRI, mostly at yield levels of 4±7 Mg ha ÿ1
with recommended rates of N, P and K applied. Statistically signi®cant yield declines at p0.05 were observed in only two of 21 data sets outside of IRRI, although all three LTEs at IRRI showed yield declines. One dry season
experiment showed a statistically signi®cant yield increase. In the less favorable climatic season (wet season), only two of 14 data sets outside of IRRI showed a statistically signi®cant declining yield trend, while three of four at IRRI showed statistically signi®cant yield declines. In rice-upland crop (typically wheat) LTEs, a signi®cant yield decline for rice was found in only one of ten data sets. For wheat, there were no statistically signi®cant yield declines, although there was one
statistically signi®cant yield increase. There was a negative correlation between the magnitude of yield decline and the level
of initial yields, but it was not statistically signi®cant. The data suggest that while yield declines exist in some LTEs, they
are less common than previously thought, particularly at moderate yield levels